Telephone box earthing

[RF Lighting]

I've been asked to look at a particularly unusual job by a local parish council.

They have taken control of a former BT red telephone box in their village.

The plan is to refurbish the box and equip it with a defibrillator and a maintained emergency light.

Having had a look at the supply today, it is an unmetered PILC supply into a street lighting style cutout.

What surprised me is that the supply and the cast iron telephone box are not earthed, and this appears to be a deliberate situation.

I've only installed one defibrillator previously, and this required an earth as the enclosure contained a class 1 heater, and I expect the majority of emergency lights would require an earth too.

I think my only option here is to install a rod and make this a TT supply.

Would it be the right thing to do to earth the telephone box, or would it be best to try and keep the entire installation as class 2?

This is certainly a new one on me!

 

[Lectrician]

They have even used plastic accessory screws!

If it is a PILC cable, is a TN-S earth connection available? Would it be best to use that and earth the kiosk?

 

[Lectrician]

Have a read of 559.10.3.2 onwards. Telephone kiosks are referenced. It seems BT chose to use double/reinforced insulation, but it is not a requirement, it is an option.

 

[bernardgreen]

Historically bringing the mains "Earth" into a phone box ( kiosk ) was found to induce hum on the telephone speech. Hence the mains to the lamp was free of the supply Earth and well protected from mechanical damage. ( adequate in the days before aggressive vandalism ).

If there is no telephone line connectiom to the kiosk then the method of Earthing would not be critical. If there is an alarm circuit then a local ground rod and RCD may be the best option for earthing and protection The alarm circuit triggers an automatic alarm at an emergency service control room if the de-fibrillator is taken or there is vandalism

 

[ban-all-sheds]

Would it be the right thing to do to earth the telephone box, or would it be best to try and keep the entire installation as class 2?

N is connected to earth.

And I don't see how you can ensure that a metal structure fixed to the ground will never be.

 

[bernardgreen]

that a metal structure fixed to the ground will never be.

It does ground the box, to a local GROUND which is normally electrically quiet if not silent. Any "Earth" bought to the kiosk in a cable that also has the electrically noisey Live and Neutral conductors will not be electrically quiet.

 

[JohnW2]

N is connected to earth. .... And I don't see how you can ensure that a metal structure fixed to the ground will never be.

I'm rather surprised that bernard has not been talking about TN-C-S earths and 'lost neutrals'. Unless the path from the metal kiosk to (true) ground had a very low resistance/impedance (presumably far from guaranteed), if the kiosk had a TN-C-S earth (with kiosk bonded to it), then in the rare scenario that bernard usually considers, one could end up with a large metal structure in the street which was at a substantial potential above true earth, waiting to be touched by people standing on the surrounding, possibly wet, ground. Admittedly that 'local' ground would rise in potential due to to being in contact with the kiosk, but not necessarily enough to limit the 'touch potential' to an acceptable level.

Kind Regards, John

 

[bernardgreen]

I'm rather surprised that bernard has not been talking about TN-C-S earths and 'lost neutrals'.

The risk of mains born noise interfering with the speech quality meant that kiosks did not have the mains "earth" imported into the kiosk

 

[bernardgreen]

Would it be the right thing to do to earth the telephone box, or would it be best to try and keep the entire installation as class 2?

Having had a look at the supply today, it is an unmetered PILC supply into a street lighting style cutout.

Of no relevance whatsoever as this is not a structure containing a telephone.

The history is relevent as to why the installation is not earthed.
There may be a phone line if a "defib is use" and/or vandal alerting system is installed.

 

[JohnW2]

The risk of mains born noise interfering with the speech quality meant that kiosks did not have the mains "earth" imported into the kiosk

It did - when it was used for its original purpose. However, RF's kiosk may well not contain any telecoms equipment ** but probably does need an earth of some sort (for Class I equipment).

The question (now) is therefore not about whether there should be an earth, but about what sort of earth RF should use.

Kind Regards, John
Edit: ** even if it does have a telephone line for 'signalling' purposes, we all have telephone lines in houses with earths, and manage to communicate via them!

 

[securespark]

Of no relevance whatsoever as this is not a structure containing a telephone.

He did say "did".

 

[securespark]

Thank God this thread doesn't involve RFI....

 

[bernardgreen]

we all have telephone lines in houses with earths, and manage to communicate via them

Huummm Well yes that is true. but then few houses are completely surrounded by a metal cage ( yes a kiosk is a wide aperture Faraday cage ) And a bit of hum on the line can be fully acceptable to people used to the poor speech quality of some mobile phones conversations.

 

[JohnW2]

Huummm Well yes that is true. but then few houses are completely surrounded by a metal cage ( yes a kiosk is a wide aperture Faraday cage ) And a bit of hum on the line can be fully acceptable to people used to the poor speech quality of some mobile phones conversations.

No argument about that, but I seriously doubt that the amount of hum would be such as to prevent reliable 'signalling' via a telephone line (if there is going to be one).

In any event, as I said, RF seemingly has no choice. He will have at least one (probably more) items of Class I equipment in the kiosk, so must have an earth, which in turn will have to be bonded to the kiosk - 'hum' or no hum. So, as I said, his question relates to what earth to use, not whether to have an earth.

Kind Regards, John

 

[jg321]

I did mention that an "earth" derived from the incoming mains Neutral could be a source of noise if there was a wired telephone line to the box and that line was used for speech based communication.

And for the record I was very interested in this information, which is on-topic and relevant IMO.

Cheers,

Jon